Introduction

This Study Guide has been designed to give you all the information needed to start controlling as a Tower controller on the VATSIM network.

Radio Communication - Basics

Because communication is crucially important for Air Traffic Control a fixed format and syntax is used, in order to minimize the risk of misunderstandings and to keep messages short. Worldwide English is the primary language in use, however in most countries you are also allowed to use the local language. In Austria VFR flights can choose their language whereas IFR flights are mostly conducted in English. Link: Buchstabiertabelle

Basic Rules

In order to achieve the goals set above the following rules important:

Listen before you talk

It's impossible for two radio stations to transmit on the same frequency at the same time. If this is done, the radio signal will be blocked and this will result in a nasty noise on the frequency. Therefore it's important that every station monitors the frequency for about 5 seconds before transmitting, to make sure there’s no ongoing radio traffic. If you hear an ongoing conversation, wait until the conversation is over before you begin to transmit. Don’t start your communication if there is a read-back expected on the last transmission even if there is a short pause.

Think before you talk

The radio traffic flow should be as smooth as possible. To achieve this it's vital to "think first" before transmitting so that a clear, concise and uninterrupted message can be sent.

As far as possible use standard phraseology and syntax

To prevent misunderstandings and to maintain the radio traffic as effective as possible, stick to standardized phraseology and skip slang and of course private messages.

Callsigns and Initial Contact

Every participant on the network has his own Callsign. Controller Positions are identified by their location and their Function (e.g. Wien Radar, Graz Tower), Aircraft either by their Registration (e.g. OE-ALB) or an Airline Callsign followed by a combination of numbers and letters (e.g. AUA25LM, SWR387).
To pronounce these letters and digits the ICAO-Alphabet is used.
To initiate the contact between two stations an initial call has to be made. This call has the following structure:

Station 1: Station 2, Station 1, Message
Station 2: Station 1, Station 2, Message

In Subsequent calls the calling station part can be ommited.
When a controller (or aircraft) transmits a message to a station it is very important that the receiving station acknowledge the message and reads back any required parts.. If the receiving station does not acknowledge, the transmitted message is considered as a lost transmission and the sender should resend the message or check if the receiving station got the message.
Items that must always be read back in full are all clearances (including altitudes, heaings, speeds, radials etc), runway in use, altimeter setting (QNH or QFE) and transition level, and all frequencies. For a controller, this is extremely important to remember, since if a pilot's readback is incorrect, the controller has to ask for confirmation, i.e a new readback. There are also items that should not be read back to reduce unnesessary radio transmissions. In short, this includes everything not mentioned above, but a few examples are: wind, temperature and other weather information (except altimeter settings) and traffic information in detail.
When giving an instruction the Callsign is stated at the beginning, when reading back you usually add it at the end of your transmission (although you are allowed to do it at the beginning too).

Aircraft and basic Flying Principles

Producing Lift

For an aircraft to fly the lift force produced by (mostly) the wings has to outweigh the gravitational force that affects the aircraft.

Basically a wing produces lift by deflecting the air it moves through into one direction. According to Newton's third law of motion the lift is produced into the opposite direction. This lift grows with the speed the aircraft has in relation to the air and with the angle the wing draws with the direction of movement. This angle is called Angle of Attack (AoA).

The principle only works as long as a steady airflow around the wing exists. As soon as the airflow seperates from the wings surface the lift starts to decerease. The AoA at which this occurs is called critical Angle of Attack. It depends on the profile of the wing and it's dimensions but for subsonic aircrafts it typically lies between 8 and 21 degrees.

Think of an level flying aircraft that reduces it speed. In order to compensate the reducing lift the pilot has to raise the nose. However at some point the Angle of Attack will cross the critical angle of Attack and the pilot will find himself in a stall. So the speed of an aircraft is limited on the lower side by the so called stall speed. Because the stall speed depends on the profile most aircraft are equipped with devices that alter the profile during flight such as flaps or slats.

On approach pilots have to fly in a certain speed range in order to conduct a safe landing. The lower boundary is called landing reference speed and is often a fixed multiple of the stall speed. As a result of this the approach speed also depends on weight an aircraft configuration (Flap/Slat setting). For safety the Approach Vapp is higher than Vref and the difference depends mostly on the weather conditions.

Generally you can say that bigger aircraft also have a bigger approach speed however at some point this rule does not work anymore because the Vref depends largely on the aircrafts weight in relation to it's maximum takeoff weight (MTOW). The speed ranges from 50 knots in a C150 up to 170 knots with a fully loaded 747. However for example it is possible that a light 747 is slower than a fully loaded 737.

Aircraft Categories

The most important ways of categorizing aircraft in aviation are by weight or by approach speed.

Weight Categories

Aircraft are categorized into three weight categories:

Category

MTOW

Light Aircraft (L)

< 7 000 kg

Medium Aircraft (M)

7 000 – 136 000 kg

Heavy Aircraft (H)

>136 000 kg

You can find a list of aircrafts in this link [1]Weight depicted is MTOW.

Approach Speed

Aircraft are categorized by their reference approach speed (Vref) at maximum landing weight:

Category

Vref

A

<= 90 knots

B

91 - 120 knots

C

121 - 140 knots

D

141 - 165 knots

E

>= 165 knots

METAR and TAF

How is an Aerodrome

As airports grew bigger over time also the workload for the Air Traffic Controller handling the traffic got bigger. Soon it was necessary to distribute this workload onto more than one controller in order to be able to cope with the traffic.

So the Tower Position got divided into thre basic types with different areas of responsibility.

Ground (GND), responsible for all traffic on the apron and the taxiways.

Tower (TWR), responsible for movements on the runway and within its associated Control Zone.

Because Tower and Ground controllers rely very strongly on what they see out of their window, these are the positions which are situated within the airports control tower.

Apart from that there are the controllers who manage the traffic once it has left the control zone. They are again divided into:

APP Positions, managing the traffic within the airports vicinity (the so called TMA, Terminal Area). In Austria they are situated directly at the airports.

ACC (Area Control Center, on VATSIM the abbreviation CTR is used) positions, which are responsible for enroute traffic. They reside in Vienna.

Since they all use their radar to control air traffic, they are also called Radar positions.

Working Delivery Positions

Clearance Delivery is responsible for checking and correcting flightplans of departing aircraft and issue routing clearances to them.

Flightplan Structure

Flight plans are documents filed by pilots with the local Civil Aviation Authority prior to departure. They generally include basic information such as departure and arrival points, estimated time en route, alternate airports in case of bad weather, type of flight (whether instrument flight rules or visual flight rules), pilot's name and number of people on board.
For IFR flights, flight plans are used by air traffic control to initiate tracking and routing services. For VFR flights, their only purpose is to provide needed information should search and rescue operations be required.

Aircraft routing types used in flight planning are: Airway, Navaid and Direct. A route may be composed of segments of different routing types.

Airway: Airway routing occurs along pre-defined pathways called Airways. Mostly aircraft are required to fly airways between the departure and destination airports. The rules cover altitude, airspeed, and requirements for entering and leaving the airway (SIDs and STARs).

Navaid: Navaid routing occurs between Navaids (short for Navigational Aids) which are not always connected by airways. Navaid routing is typically only allowed in the continental U.S. If a flight plan specifies Navaid routing between two Navaids which are connected via an airway, the rules for that particular airway must be followed as if the aircraft was flying Airway routing between those two Navaids. Allowable altitudes are covered in Flight Levels.

Direct: Direct routing occurs when one or both of the route segment endpoints are at a latitude/longitude which is not located at a Navaid. This is a routing from Vienna

Issuing IFR Routing Clearances

DEL gives routing clearances to all departing aircraft with the following information:

Destination of aircraftSID (= Standard instrument departure) Normally the filed SID is given
Initial climb altitude after departure (5000ft)
Squawk (Squawk assignments for LOWW are 4600 to 4620)
QNH (Local QNH of airport according to latest METAR) = given with taxi clearance
CTOT (= Calculated take-off time) Slot time (Normally not used on the VATSIM network)

Some Aircraft are not able to follow SIDs for various reasons, most of the time due to missing equipment. In these cases you should issue a so called vectored departure. A vectored departure clearance includes the same components as a normal clearance but instead of the SID you issue instructions to be carried out after departure. In this case the initial climb altitude is mandatory.

You can find the instructions for each Airport within the Study Guide:Airport Details
If the pilot responds with a correct readback you should answer with the following phrase:

Callsign, readback correct.

Afterwards you either hand the pilot over to GND or wait for his startup request, depending on local procedures.

Special Situations (High Traffic, Slots, ...)

Slots

In order to guarantee a safe flow of traffic and to minimize delays in the air so called slots are being used. A slot is a timeframe of five minutes before to ten minutes after the CTOT mentioned before. The aircraft has to depart within this timeframe from its departure airport.
On the VATSIM network this system is only used on special occasions.

Verhalten in Situationen mit erhöhtem Verkehrsaufkommen

Sometimes one of your neighboring sectors has to stop accepting traffic. In these cases you should delay an aircrafts start-up clearance.

Air-taxiing is the Movement of a helicopter / VTOL above the surface of an aerodrome, normally in ground effect and at a ground speed of normally less than 20 KT (37 km/h). Please Note: The actual height may vary, and some helicopters may require air-taxiing above 25 FT (8 m) AGL to reduce ground effect turbulence or provide clearance for cargo sling loads.

Ground Traffic Management

To organise the traffic on ground different techniques are available, some of them relying on the pilots seeing each other. Generally you should avoid clearing two aircraft onto crossing pathways, unless you are sure they will never meet each other. To achieve this you should instruct aircraft to hold short of taxiways in the way stated above. Consider the following situation:

You are the Ground Controller at Vienna Airport. Runways active are 34 for landing and 29 for departure. DLH6KM has vacated rwy 34 and requests taxi to its parking position. LZB421 is ready for taxi at stand 7Q.

GND:LZB421, give way to the DLH B737 crossing left to right on L, thereafter continue
taxi to holding point runway 29 via taxiways Exit 2, M and A1.
LZB421:Giving way to the 737 from left to right, then continuing taxi to holding point
runway 29 via Exit 2, M and A1.

Of course you have to make sure that this instruction is unambiguous, so there shouldn't be two DLH B737s in the area. Also in low visibility operations this procedure might not work very well, in this case you might have to give the aircraft the instruction to continue taxi when the other aircraft has passed.
In some cases it is also useful to let one aircraft follow the other:

GND:LZB421, follow the Austrian DASH 8 crossing you right to left on M to holding point runway 29.
LZB421:following the DASH 8 crossing us right to left on M to holding point runway 29.

Intersection take-off

Some flights do not need the whole length of their given departure runway so they might request takeoff from an intersection somewhere down the runway. This procedure is called a intersection takeoff. You should only grant this in coordination with Tower and if traffic situation permits.
Also at some airports intersections are used to be more flexible in the departure sequence (see section Departure Seperation).

Special Situations (High Traffic, Slots, ...)

Slots

In case the above mentioned slot regulations are in force ground has the responsibility to set up a departure sequence in a way that the aircraft do not miss their slot.

Opposite runway operations

At some austrian airports it is very common to use opposite runway configurations (departure and arrival runway are opposite to each other). In these situations it can happen very fast that you have two aircraft facing each other nose to nose. Special attention should be paid to avoid this situation.

Working Tower Positions

Tower is responsible for all movements on the runways as well as for all movements within the control zone (CTR), (10NM radius, GND to 2500ft MSL). Tower is also responsible for ground and delivery if they are not online. He also decides which runways are in use.

ATIS

ATIS stands for Automatic Terminal Information Service and is a usually automatically generated broadcast that contains essential informations for pilots. It is continuously broadcasted on a dedicated frequency. On initial contact with the controller, pilots should already have listened to the ATIS and state the identifying letter.

A ATIS broadcast has to consist of:

Name of the Airport

Identification Letter

Time of Observation

Active Runways

Transition Level

Wind direction and velocity

Visibilities

Special weather conditions (such as rain)

Cloud ceiling

Temperature and Dewpoint

QNH

Trends

It is updated every 30 minutes or as soon as significant changes occur.

Determination of active Runways

Pilots normally prefer to takeoff and land the aircraft with the nose into the wind because it shortens the Rwy length required to safely operate the aircraft. The wind direction given in the METAR is the direction the wind is coming from, so it is easy to compare this wind to your given runways. Example:

You are the Tower controller at Salzburg Airport. The only runway at Salzburg is runway 16-34 so you have two directions available (roughly 160° and 340°.) The wind is coming from 180° at 5 knots. So the usual Runway in use would be rwy 16 for takeoff and landing.

However, at most airports a preferred runway configuration is defined (Find them here: Study Guide:Airport Details) which should be used if traffic situation and weather permits. Aircraft have certain limitations they can operate in, so normally the tailwind component should not exceed 5-10 knots (again depending on airport). Also the allowed crosswind is limited (This depends very much on the aircraft).
Be aware that it is the pilots responsibility to accept a certain wind component and that this decision is often based on performance issues, so one pilot might accept the next one refuses to take a certain runway.

So back to our example above:

At Salzburg, due to the terrain in the vicinity and city of Salzburg around the airport, runway 34 is preferred for departures and rwy 16 for landing. So the indicated configuration would be DEP 34, ARR 16.

Transition Altitude/Transition Level

Knowing the altitude you are flying is one of the most important informations you need in order to safely operate an airplane. Aircraft Altimeters use the air pressure around them to determine their actual altitude. In order to get correct readings you have to use the actual local pressure in your area. As a memory hook you can use this: The altimeter needle moves in the same direction you turn the rotary knob to adjust the pressure. If you turn it counterclockwise, the needle also turns counterclockwise and therefor indicates a lower altitude.

On the other hand it would not be very practical to use the local pressure while flying at higher altitudes, since terrain is not an issue here and you would have to set a new pressure setting in your altimeter every few minutes.

To avoid this pilots use the local pressure when departing from an airport until they pass the so called Transition Altitude (TA), where they set the so called standard pressure (QNH 1013 hpa or Altimeter 29.92 inHg). They continue to use this setting until they descend through the Transition Level (TRL) at their destination airport (or an airport on their route), where they set the local pressure again.

In airport charts only TA is given, whereas TRL has to be determined by ATC. Use the following table to calculated your TRL:

The room between TA and TRL is called Transition layer. It ensures that the minimum spacing of 1000 ft between aircraft flying in lower part (with local pressure) and the upper part (using Standard pressure).

Runway Separation

The runways are one of the most dangerous spots on an airport because aircraft are travelling at high speed with little room to maneuver and most of the time no ability to stop at a reasonable distance. Because of this the general rule is that only one aircaft may be cleared to use a runway at the same time. What this means practically and exceptions from this rule are explained in the following chapters.

Departing Traffic

So now we are at the point where the pilot reaches the Holding Point of his departure runway and reports ready for departure. What are the things you should check before issuing the takeoff clearance?

Have a look at the flightplan. Take note of the type of aircraft and the Departure Route.

Check the traffic approaching the runway.

To give him the takeoff clearance the following phrase should be used:

The pilot lines up on the runway, advances the throttle and takes off. When he is well established in climb check he is squawking Mode C and the right Code. Afterwards he is handed off to the next Controller, in this case a radar position:

The next aircraft reports ready for departure. Again check the points above, but this time we cannot give the takeoff clearance straight away because the preceeding aircraft is still occupying the runway. Now you get to know the first exception to the Runway Seperation rule above. To speed things up you can instruct the next aircraft to line up behind the first one while this one is still in the takeoff roll occupying the runway:

TWR: AZA639, behind departing Austrian Airbus A319, line-up rwy 29 behind and wait.
AZA639: behind departing Airbus lining up runway 29 and waiting behind, AZA639.
Note: The two times behind in this instruction is not a typing error but was implemented
to emphasize that part of the clearance.

This type of clearance is called a conditional clearance. The earliest possible point where you can issue the next takeoff clearance is, when the preceeding aircraft has overflown the opposite runway end or has clearly turned onto either side of it. However in some cases this could be very close which leads us to the next chapter but before lets have a look on helicopters.

Helicopters are sometimes able to start from there current position like a Helipad or a normal stand, if he want to depart from a Runway you can use the normal Phrases for VFR Traffic.

Departure Seperation - Based on Type of Aircraft and departure route

One of the main tasks of air traffic control is to keep aircraft at a safe distance to each other. So imagine the following situation:

Two aircraft are departing right after each other.

The first aircraft is a relatively slow Cessna 208 (~around 70 knots in climb), the second one a fast Boeing 767 (140-180 knots on the initial climb).

Both follow the same departure route.

Obviously it would not take long until the B767 catches up with the Cessna, a potentially very dangerous situation! You can see, that it is very important to check the flightplan of the aircraft you are about to clear for takeoff.
The minimum radar seperation in the area around an airport is 3 nm or 1000 feet. These are the limits radar stations have to obey. Tower Controllers should aim to achieve the following seperation for departing aircraft following departure routes which share a common part:

Fast followed by slow

3 nm

Matching Types

5 nm

Slow followed by fast

10 nm

In extreme examples like the one above it is often more advisable to coordinate with APP to find another solution. Often this involves clearing the aircraft to a non standard altitude or departure route:

The other main task of ATC is to expedite the flow of traffic. Situation:

You have numerous aircraft departing from the same runway, following different departure routes. Some of them involve immediate right turns other SIDs immediate left turns.

There are two holdingpoints available.

It would benificial to use the gaps that arise between the aircraft using similar Departure Routes, so in close coordination with ground you should try to distribute aircraft over the holding points in a way to be able to fill those gaps.

Departure Seperation - Based on Wake Turbulence Category

There are two ways aircraft influence the air around them when passing through it:

Jetwash produced by the engines

Turbulence created at the wings and especially at the wingtips

This turbulence can cause severe problems or even loss of control for following aircraft.
The wake turbulence categories are based on the Maximum Takeoff weight (MTOW) of the aircraft:

Light Aircraft (L)

< 7 000 kg

Medium Aircraft (M)

7 000 – 136 000 kg

Heavy Aircraft (H)

>136 000 kg

For departing aircraft, 2 minutes separation (3 minutes if the succeeding aircraft departs from an intersection) is applied when an aircraft in wake turbulence category LIGHT or MEDIUM departs behind an aircraft in wake turbulence category HEAVY, or when a LIGHT category aircraft departs behind a MEDIUM category aircraft.
You may issue a take-off clearance to an aircraft that has waived wake turbulence separation, except, if it's a light or medium aircraft departing as follows:

Behind a heavy a/c and takeoff is started from an interception or along the runway in the direction of take-off.

Behind a heavy a/c that is taking off or making a low or missed approach in the opposite direction on the same runway.

Behind a heavy a/c that is making a low or missed approach in the same direction of the runway.

To point out this hazard to a pilot the following phrase should be used:

During periods of high traffic it is likely that you have more than one aircraft approaching the same runway at the same time. Approach has to ensure the minimum radar seperation of 3 nm and additionally increased seperation due to wake turbulence.

To give you an idea how dense traffic can get in real life consider that during peak times and good weather the seperation is reduced to 2,5 nm. This equals to one landing every 75 seconds. However on VATSIM the minimum seperation is 3 nm which already requires good cooperation from all the pilots involved.

Merging Departing and Arriving Traffic

And now to the most fun part of being a Tower Controller. Sometimes you get into the situation that you use the same runway for departures and arrivals. Either your airport has only one runway or weather demand this configuration.

Still the above rule of only one aircraft at the same time applies, however we also use conditional clearances which look very similar to those above in the departing traffic section.

To avoid misunderstandings, this time we make sure that the Pilot has the the landing aircraft in sight.
You don't have to worry about wake turbulence seperation between landing and departing aircraft since they never cross through each others wake.

To depart an aircraft in front of an approaching aircraft at the time of the departure clearance given the arriving aircraft should not be closer than 4 nm to touchdown.
To squeeze a departing aircraft between two arrivals you normally need a minimum of 6 nm between them. It is important for you to check carefully if you have the necessary gap, so have a close look at the distance between the arrivals and their speed. If the second one comes in faster than normal consider this in your calculation. Also you should make sure, that the pilot will be ready for departure when you need him to depart. To check this use the following phrase:

Callsign, are you ready for immediate departure?

Again it is a good idea to give the pilot an idea of the traffic situation around him.

Example:

You are the Tower Controller at Vienna airport. Runway 29 is active for departures and arrivals. One aircraft is on a 5 nm final, one at 12 nm out. Additionally you have two departures waiting at the holding point of ruwnay 29.

VFR Traffic - Differences

The essential collision safety principle guiding the VFR pilot is "see and avoid." Pilots flying under VFR assume responsibility for their separation from all other aircraft and are generally not assigned routes or altitudes by air traffic control. Governing agencies establish specific requirements for VFR flight, consisting of minimum visibility, distance from clouds, and altitude to ensure that aircraft operating under VFR can be seen from a far enough distance to ensure safety.

To guide VFR TRaffic through your airspace you make use of VFR Routes, Sectors and reporting Points.
Used phrases:

When using an extended downwind you should always consider that the aircrafts speed might be considerably lower than the speed of other aircrafts involved. So if an aircraft has to fly a long way out it might take some time for it to come all the way back, generating a big gap in the arrival sequence. Instead you should aim to keep the plane within the vicinity of the airfield:

TWR: OE-AGA, Make a right three-sixty.
OE-AGA: Making three-sixty to the right.
TWR: OE-AGA, Orbit left
OE-AGA: Orbiting left, OE-AGA

The second instructions means, that the pilot should make orbits until further advice.

Information Positions

Special Situations (High Traffic, Slots, ...)

High traffic situations

During high traffic situations communication with adjacent approach sectors is very important. Especially during single runway operations you might have to ask for increased inbound spacing to be able to fit in departing aircraft.

Opposite runway operations

This is one of the more difficult situtions for a Tower controller. You have to consider the departure route of each aircraft to estimate the required spacing to arriving traffic. Again close coordination with approach is very important.